1. Field of the Disclosure
The present disclosure relates to a rotary blade and a method of designing and manufacturing a rotary blade, such as a rotary blade for use in a gas turbine engine.
2. Description of the Related Art
It is desirable to reduce the clearance between the tip portion of a rotary blade and the casing of a gas turbine engine in order to maximise fuel efficiency.
However, reducing the build clearance between the tip portion and the casing can lead to undesirable rubbing of the tip portion on the casing during engine running which can induce excessive stresses in the rotary blade. In addition to radial growth due to centrifugal force, such rubbing occurs, in part, because the tip portion twists during running of a gas turbine engine due to loads generated by the gas flow and rotary motion.
Blade tip portions are typically formed to be concentric with the casing e.g. by grinding or EDM wire-cutting the tip portions whilst rotating a drum or disc (on which the rotary blades are mounted) and the attached rotary blades at low speed. This gives good circularity about the rotational axis i.e. the tip portions are axisymmetric about the engine axis.
Casings e.g. compressor casings, have a radially inner surface (facing the blade tip portions) which is typically cylindrical or conical.
Tip portions that are axisymmetric about the engine axis (i.e. concentric with a cylindrical casing) in their cold/static position, may rub the casing at both their leading and trailing edges as the clearance between the leading and trailing edges and the casing will reduce as a result of blade twist during running of the turbine engine (assuming that the axis of twist is between the leading and trailing edges). This is shown in FIGS. 2a and 2b by way of example.
Blade tips that are concentric with a converging conical casing in their cold/static position may rub the casing at the leading edge as the clearance between the leading edge and the case will reduce as a result of blade twist during running of the turbine engine (assuming that the axis of twist is between the leading and trailing edges). Conversely, the clearance between the trailing edge and the casing will increase as a result of the blade twist resulting in reduced aerodynamic efficiency. For example, a large aerofoil in a gas turbine compressor develops a blade tip twist of around 3° during engine running resulting in the clearance between the leading edge and the casing decreasing by around 0.4 mm and the clearance between the trailing edge and the casing increasing by around 0.7 mm.
The rubbing can cause blade damage both at the tip portion (resulting from the direct contact with the casing) and throughout the rotary blade (resulting from stresses arising from vibrations and thermal damage arising from frictional heat caused by the rubbing). This rubbing can cause significant reduction in fatigue life on a blisk (a rotary disc with integrally-joined rotary blades) since the rotary blades will bend at the interface with the disc—this area is typically highly stressed during engine running. Furthermore abraded material can block cooling holes in the downstream turbine aerofoils.
In order to avoid the heavy rubbing during running conditions, the build clearance between the blade tip portions in their cold/static state and the casing is typically increased. This reduces the turbine efficiency especially in the early stages of engine running.
It is known to provide a track liner on the interior surface of the compressor casing to provide a shroud for the tip portions. The track liner may be formed of an abradable material (e.g. an epoxy resin) which is abraded by the blade tip portions to form channels in the track liner into which the tip portions extend. These abradable track liners have found some success with metallic rotary blades (e.g. titanium and nickel rotary blades) which are very durable but a disadvantage of these track liners is that the running clearance is set by the longest rotary blade. These track liners are not appropriate for use with rotary blades formed of composite material (e.g. fibre-reinforced plastic material) which are less durable and readily suffer damage to the tip portions.
Ablative casings are also known where the tip portions of the blades are formed of an abradable material and are abraded by the casing during rubbing. Again, damage to the rotary blade can occur as a result of vibrational and thermal energy arising during rubbing.